It is well known that the DC produced by a generator having one pole grounded propagates through the ground and may give rise to changes in the electrical state of buried works, and may degrade these works by electrolysis. Thus a buried structure connected to a DC generator producing a stable electrical current, e.g. a conventional cathodic protection device, or a DC electrified path producing stray currents, give rise ot interactions with adjacent buried metal structures, i.e. they change the potential of the metal structures as measured relative to the medium in which the structures are placed due to various current interchanges passing through the surrounding medium into the structures or from the structures into the surrounding medium. The magnitude of the current density depends on the size of the bare metal area of the structure which is in contact with the surrounding medium. The higher the current density, the greater the interaction effect of the current on a given structure.
The interactions due to direct current flows in the ground can cause a buried structure to have detrimental effects on an adjacent buried structure. Proposals have already been made to show up such interactions by monitoring the potential of each of the structures, which structures may be buried ducts, for example, by using voltmeters having high internal resistance or voltage recorders and non-polarizable electrodes placed in the ground in the vicinity of the structures. For performing the measurements, a voltmeter or a voltage recorder is connected between each of the structures and the corresponding reference electrode, and a plurality of measurements are preformed, with the DC installations being successively switched on and off. Prior methods and apparatuses for determing interactions due to direct currents are based essentially on measuring the variation of the potential of the metal in a buried structure relative to the medium in which the structure is placed. However, in practice it is difficult to interpret the measurements relating to interference between adjacent works situated in the same surrounding medium, and indeed, it is sometimes practically impossible to determine which is the interfering work and which is the work suffering interference.
Errors of interpretation are due mainly to the following three factors:
1. measurement of the duct-to-ground potential which takes account to the voltage drop in the medium caused by the flow of current;
2. the resistivity of the medium which in part determines the chemical properties of the electrolysis; and
3. the coatings of the structures which isolates them to a greater or lesser extent from the medium.
Even if the presently-favored methodology did not lend itselt to errors of interpretation, it would nevertheless lead to curative dispositions being taken but never to preventative dispositions in the event of subsequent accidental damage to the coating (or on the appearance of microcracks due to aging).
An object of the present invention is to remedy to above-mentioned drawbacks and to make it possible to monitor in reliable manner ther interactions due to direct currents on adjacent buried metal structures.
More particularly, the present invention seeks to make it possible to clarify the quality of preventative cathodic protection and to evaluate the possible interference from adjacent works, and also to verify the interference from stray currents.
Another object of the invention is to provide a method and apparatus for determining interactions accurately, reliably, easily, and enabling good-quality diagnoses to be made.